Arthur has a minor magical ability: he can hear people’s thoughts when he touches them (unluckily for him, he never learned to control it so he gets this overwhelming and painful rush of thoughts whenever he touches someone… but as the crown prince he can usually avoid touching people and he always wears gloves).
BUT THEN he meets Merlin, his sassy and clumsy manservant. He doesn’t let Merlin dress him at first, too scared of the barrage of thoughts/feelings/memories/emotions. But one day Merlin trips and Arthur instinctively catches him, accidentally brushing his skin. He braces himself for a loud flood of Merlin’s thoughts, but instead all Arthur can sense is just a 'wall' and he can’t hear anything beyond it. So he starts becoming super handsy with Merlin to try and figure out why Merlin is different. Arthur has never had to wonder what someone was thinking before, but Merlin is a complete mystery to him.
Cue: Arthur watching Merlin closely to learn his mannerisms and facial expressions, Arthur asking Merlin about his life back in Ealdor, Arthur falling in love in the process.
BUT LIKE ALSO Arthur is touch-starved because of his gift, and suddenly Merlin is the one person he can touch without being completely overwhelmed with their toughts, and Arthur takes every excuse to manhandle touch Merlin because he's never been able to be physically affectionate before.